Thermoelectric composition

ABSTRACT

A P-type, thermoelectrically useful alloy composition consisting of copper, silver, and either tellurium or selenium.

Ufited tates Maui [191 Ham 1 Jr. Dec. 10, 1974 THERMOELECTRIC COMPOSITION [56] References Cited [75] Inventor: Edward F. Hampl, Jr., Cottage [UNITED STATES PATENTS Grove, Minn. 2,397,756 4/1946 Schwarz 136/238 1 5 F [73] Assignee: Minnesota Mining and Z 3 3 22 Manufacturing Company, St. Paul, 2 970 24 1 19 1 Suhagun Minn. 3,045,057 7/1962 Cornish 3,095,330 6/1963 Epstein... [22] plied 1973 3,132,488 5/1964 Epstein 136/238 [21] Appl. No.: 321,222

Primary Examiner-Carl D. Quarforth Related U.S. Application Data Assistant l Hunt Continuation of 635,948, April 1967 Attorney, Agent, or FirmAlexander, Sell, Steldt &

abandoned, which is a continuation-in-part of Ser. DeLahunt No. 463,148, June 11, 1965, abandoned.

57 AB TRA T [52] U.S. Cl 136/238, 136/241, 75/134 1 C [51] Int Cl Holv 1/15 A P-type, thermoelectrically useful alloy composmon [58] Field Consisting of copper, silver, and either tellurium or se- 252/514 lenium.

l9'Claims, 8 Drawing Figures usefulness as a thermoelectric converter is generally regarded as the thermoelectric figure of meritZ, which is defined as where S( T) is the Seebeck coefficient, p( T) is the electrical resistivity, and K( T) is the thermal conductivity, all of these parameters being functions of temperature as indicated. Thermoelectric conversion efficiency (the ratio of electric energy output to thermal energy input for a leg of the material in a thermoelectric generator, for example) may be calculated from Z as shown by the following Ioffe expression for efficiency:

in which Z is the average value of Z in the temperature interval T to T,,, and the latter are the absolute temperatures of the cold and hot junctions, respectively. Another indicator, often used instead of figure of merit characterizing the thermoelectric value of a material, independent of the materials thermal conductivity, is the power number which is defined as Power Number [S(T)] /p(T) and which provides a relative measure of the electric power-producing capability of the material.

Inherent in evaluations of thermoelectric materials, as in evaluations based on the above expressions, are the temperatures at which the materials are useful. The available thermoelectric materials that have the highest values of thermoelectric conversion efiiciency are generally limited to use below the rather moderate temperatures of 500-600C. Above such temperatures these materials become intrinsic electrical conductors, and increasing the hot-junction temperature of thermoelectric legs of such prior art materials above 500600C. results in lowering the overall thermoelectric conversion efficiency of the leg. Further, most of the prior materials of useful efficiency deteriorate at high temperatures by sublimation, for example with a resultant loss of properties. These temperature limitations in prior materials have prevented the improvement in thermoelectric conversion efficiency that it is recognized could be achieved by heating the hot junction of a thermocouple to higher temperatures.

In contrast to the prior art compositions, the products of the present invention remain extrinsic and exhibit excellent thermoelectric properties over a broad temperature range that extends to temperatures of 9001,l50C. The new compositions have values of figure of merit and conversion efficiency that are superior to those of prior art compositions both in the lowtemperature portions (i.e., less than 500600C.) and in the high-temperature portions of these large thermal gradients. Further, the new compositions are chemically stable and show little sublimation at high temperatures.

An additional major advantage that the new compositions have over prior art thermoelectric materials of useful efficiency is a much higher mechanical strength. Because of the rather low tensile and shear strengths of such prior materials, thermoelectric legs are mounted in conventional thermopiles by complex arrangements which, though adding weight and size and encumbering use of the device, limit the, formation of destructive tensile and shear forces that would otherwise arise within the legs in the environment of a thermal gradient. With the introduction of the compositions of this invention, which have superior tensile, shear, and compressive strengths, the thermoelectric device designer is freed from'many of the past mounting requirements.

In general, the new compositions are alloy compositions consisting of the constituents copper, either tellurium or selenium, and, in preferred embodiments, silver. Tellurium or selenium is included in amounts between about 32.5 and 33.7 atomic percent of the total composition. In the copper-silver-tellurium compositions, copper is included in amounts between about 27 and 67 atomic percent, while in the copper-silverselenium compositions, copper is included in amounts between about 60 and 67 atomic percent. Essentially the balance of the compositions is silver, the maximum amount of silver being about 40 atomic percent in the telluride compositions and about 7 atomic percent in the selenide compositions. In addition to these elements, modifying agents that enhance P-type thermoelectric properties may be included in typical modifying amounts.

With changes in temperature, these alloy compositions undergo crystal transformations that modify their thermoelectric properties. When heated above a temperature that varies between approximately C. and 575C., depending on the particular alloy composition, the new compositions assume a stable crystal structure believed to be a cubic crystal structure. It is above this temperature, hereafter referred to as the upper temperature of transformation, and with this crystal structure, hereafter referred to as the high-temperature crystal structure, that the superior properties of the compositions are found. Below this upper temperature of transformation, depending again on'compo sition, a number of mixed crystal phases, additional temperatures of transformatiom'and generally inferior thermoelectric properties usually exist. Though the thermoelectric properties of alloy compositions that include copper, silver, and tellurium or selenium have previously been studied, to my knowledge those studies have never resulted in disclosure of the alloy compositions of this invention nor of alloy compositions at all comparable in usefulness to those of this invention. Nor has the superiority of the stable, high-temperature crystal forms of the compositions of the invention been recognized.

" In the drawings:

FIGS. 1, 2, and 3 are plots of Seebeck coefficient, S, resistivity, p, and power number, S2/p, respectively, for various copper-silver-tellurium compositions of the invention;

FIG. 4 is a ternary diagram on which are plotted the ranges of copper, silver, and tellurium in alloy compositions of the invention;

FIGS. 5, 6, and 7 are plots of Seebeck coefficient, S,

resistivity, p, and power number, SZ/p, respectively, for various copper-silver-selenium compositions of the invention; and

FIG. 8 is a ternary diagram on which are plotted the ranges of copper, silver, and selenium in alloy compositions of the invention.

The alloy compositions of the invention, which are substantially pseudo-binary compositions of Cu Te and Ag Te, or of Cu Se and Ag Se, have different crystallographic features depending on their particular composition. Though I do not wish to limit myself strictly to particular phase boundaries, which vary with temperature, it is proposed that the telluride compositions of the invention include the following essentially singlephase compositions: alpha, a primary phase having an imperfect lattice structure based on the molecular formula (Cu,Ag) Te, and including a minor second phase of copper and silver, this structure existing for compositions that comprise between about 60 and 67 atomic percent copper, up to about 7 atomic percent silver, and between about 32.5 and 33.7 atomic percent tellurium; beta, an intermediate phase existing for compositions that comprise between about 43 and 50 atomic percent copper, between about 17 and 24 atomic percent silver, and between about 32.5 and 33.7 atomic percent tellurium; and gamma, an intermediate phase existing for compositions that comprise between about 30 and 40 atomic percent copper, between about 27 and 37 atomic percent silver, and between about 32.5 and 33.7 atomic percent tellurium.

These single-phase telluride compositions are preferred telluride compositions since they have lower electrical resistivities and thus higher values of power number, figure of merit, and conversion efficiency. The telluride compositions of the invention between the single-phase compositions have mixtures of crystal structures. Though less preferred because of their higher electrical resistivity, these plural-phase compositions exhibit a Seebeck effect extending to high temperatures of the same order as the single-phase compositions, and thus are also useful. Compositions including copper, silver, and tellurium in amounts outside the ranges of about 32.5 and 33.7 atomic percent tellurium, about 27 and 67 atomic percent copper, and about to 40 atomic percent silver have inferior thermoelectric properties or have N-type properties.

Of the single-phase telluride compositions of the invention, the most preferred are the gamma phase compositions which, as noted previously, include between about 30 and 40 atomic percent copper and between about 27 and 37 atomic percent silver. These compositions have the highest figures of merit and the best conversion efficiencies. Within this range, a narrower range of compositions having the highest thermoelectric properties extends between about 32 and 36 atomic percent silver, about 31 and 35 atomic percent copper and about 33.2 and 33.5 atomic percent tellurium.

In general, the selenide compositions of the invention are single-phase compositions, though the compositions containing larger amounts of silver are singlephase only at higher temperatures. The selenide compositions have an imperfect lattice structure based on the molecular formula (Cu,Ag) Se, and include a minor second phase of copper and silver. Within the described range of selenide compositions, a narrower range of distinctly superior compositions is found including between. about 0.7 and 3 atomic percent silver,

with the most superior compositions including about 1 atomic percent silver (that is, the most superior properties are found in a range beginning at about 0.7 and extending somewhat about 1 atomic percent silver). Especially the latter selenide compositions, along with the gamma telluride compositions, are preferred compositions of the invention because they have the highest thermoelectric conversion efficiencies. The selenide compositions are somewhat more preferred because, as will be shown, they have a high melting point and assume a useful high-temperature crystal form at the rather low temperature of about C. (It is believed that these compositions undergo a crystal transformation only at this temperature, which is nevertheless referred to as the upper temperature of transformation.) Compositions including copper, silver, and selenium in amounts outside the ranges of about 32.5 to 33.7 atomic percent selenium, about 60 to 67 atomic percent copper, and about 0 to 7 atomic percent silver have inferior thermoelectric properties or N-type properties.

As noted previously, the thermoelectric properties that make the novel compositions useful occur when the compositions of the invention have the cubic crystal structure exhibited at elevated temperatures. Below the upper temperatures of transformation, the compositions exist as plural phase compositions and have poor thermoelectric properties. Of the telluride compositions, the alpha compositions begin to assume a cubic structure at about 550C, the beta compositions at about 280C., and the gamma compositions at about 185C. The alpha compositions assume a face-centered cubic structure and have an approximate lattice parameter of 6.25 A as measured for a composition comprising 60 atomic percent copper, 6.7 atomic percent silver, and 33.3 atomic percent tellurium at 750C. The beta compositions assume a simple cubic structure and have an approximate lattice parameter of 4.78 A, measured with a composition comprising 50 atomic percent copper, 16.7 atomic percent silver, and 33.3 atomic percent tellurium at 475C. The gamma compositions also assume the face-centered cubic structure and have an approximate lattice parameter of 6.36 A, measured with a composition of equal atomic percentages of copper, silver, and tellurium at 350C. The selenide compositions of the invention begin to assume a facecentered cubic crystal structure at about 95 to C. A composition comprising 65.77 atomic percent copper, 0.93 atomic percent silver, and 33.3 atomic percent selenium, has an approximate lattice parameter of 5.93 A at 600C.

The data plotted in FIGS. 1, 2, and 3 and FIGS. 5, 6, and 7 of the drawings give an indication of the superior properties possessed by the compositions of the invention. In FIGS. 1, 2, and 3, the telluride compositions represented by the curves labeled A include equal atomic percentages of copper, silver, and tellurium; those represented by the B curves include 40 atomic percent copper, 26.7 atomic percent silver, and 33.3,

atomic percent tellurium; those represented by the C curves include 44.5 atomic percent copper, 22.2 atomic percent silver, and 33.3 atomic percent tellurium; those represented by the D curves include 50 atomic percent copper, 16.7 atomic percent silver, and 33.3 atomic percent tellurium; and those represented by the E curves include 66.7 atomic percent copper and 33.3 atomic percent tellurium. In FIGS. 5, 6, and

7, the selenide compositions represented by the curves labeled M include 65.77 atomic percent copper, 0.93 atomic percent silver, and 33.3 atomic percent selenium; those represented by the curves labeled N include 65.25 atomic percent copper, 1.40 atomic percent silver, and 33.34 atomic percent selenium. The improvement of properties under high-temperature conditions is illustrated in FIGS. 1 and 5, which show the Seebeck coefficient for representative compositions of the invention, and the sharp upward breaks in the Seebeck coefficient that occur at the onset of the upper temperatures of transformation. The representations in FIGS. 3 and 7 of power number (S /p) show rather high values which extend to temperatures of approximately 900l,l25C., the approximate melting point of the alloys.

Beside having superior power number characteristics, the alloy compositions of the invention have thermal conductivities that are surprisingly very low, due especially to a low lattice component of thermal conductivity. As a result of their very low thermal conductivity, the new compositions have high figures of merit into high-temperature regions and, as indicated by the loffe efficiency expression set out above, correspondingly high conversion efficiencies. The thermal conductivities of the compositions have been measured as follows: the alpha telluride compositions, about 0016- 1 7 watts/cm C. at a temperature of approximately 560C; the beta telluride compositions, about 0.01 l0.0l6 watts/cm C. over a temperature range of about 300- 75C.; the gamma telluride compositions,

about 0.007-0.008 watts/cm C. over a temperature range of about 200400C.; and the selenide compositions, about 0.007 to 0.01 l watts/cm C. over a temperature range of about 125C. to 600C.

As previously indicated, both telluride and selenide compositions of 67 atomic percent copper and 33 atomic percent tellurium or selenium assume a defect crystal structure, with copper atoms being displaced from the lattice. The free copper atoms tend to agglomerate at the surfaces of the thermoelectric leg in the form of whiskers and do not beneficially contribute to the thermoelectric properties of the element. Compositions containing more than about 66.5 atomic percent copper, for example, occasionally exhibit whisker agglomeration to the extent of providing short circuiting conductive paths that detract from the effectiveness of the element. Therefore, a more preferred composition is one that is slightly telluriumor selenium-excess, that is, includes about 33.5 atomic percent tellurium or selenium and 66.5 atomic percent copper.

As silver is substituted for copper in copper telluride or copper selenide compositions, an improvement in properties is noted. With the telluride compositions a substantial improvement in Seebeck coefficient is noted in compositions including about 1 atomic percent silver or more. Accordingly, if the Seebeck voltage generated by an element is the primary consideration, an alpha-phase telluride composition that includes about 1 atomic percent or more of silver is preferred over one that does not. Asindicated the preferred selenide compositions also include about 1 atomic percent or more of silver.

The alpha-phase telluride compositions and the selenide compositions'are useful at higher temperatures than are other compositions of the invention, because of their higher melting points, For example, they can be used to temperatures 200C. higher than can the gamma telluride compositions. This usefulness at high temperatures especially adapts these compositions to inclusion in a thermoelectric leg as the hightemperature segment of the leg. Since the best results in segmenting are obtained when the segments are compatible, for example, by comprising the same ingredients, a desirable thermoelectric leg is one in which the low-temperature segment is a gamma telluride composition and the high-temperature segment an alpha telluride composition.

Completion of the change to the high-temperature crystal structure that occurs as a composition moves above the upper temperature of transformation requires a period of time that depends on the particular alloy composition and the temperature to which it is heated. The higher the temperature is above the upper temperature of transformation, the faster will be the transformation. At temperatures only slightly above the upper temperature of transformation several hours are required to develop the superior thermoelectric properties that accompany the cubic structure. Accordingly, it is preferable to use a thermoelectric leg formed of an alloy composition of this invention under noncycling or infrequently cycling temperature conditions such as are frequently found in control and nuclear power generator uses. Further, to take full value of an alloy composition of this invention, the entire length of it in the thermoelectric leg should be heated above the upper temperature of transformation. If the entire thermoelectric leg is of the gamma composition, for example, the cold junction temperature should be maintained above about 185C; if the leg is made in segments with only the high-temperature segment formed of the gamma compositions, that segment should preferably be maintained at a temperature above 185C. Similarly, the minimum temperatures within a segment or leg formed from an alpha or beta telluride composition or a selenide composition should exceed about 550C, 280C, or C., respectively.

Higher Seebeck voltages may be generated with materials of the invention that have been altered by the inclusion of modifying agents. The increase in voltage is generally accompanied by a corresponding increase in resistivity, however. I have found improvement in Seebeck voltage with such P-type modifying agents as sodium, potassium, lithium, tin, chromium, manganese, etc.; preferably these agents are added in amounts less than about one weight percent of the final composition.

.Thermoelectric legs may be fabricated from alloy compositions of this invention prepared by mixing finely divided copper, silver, and tellurium or selenium material, each containing less than 0.01 percent by weight impurity, in the desired proportions; melting the mixture; and casting the resulting melt in an appropriately shaped mold. The mixture should be melted in an oxygen-free or reducing atmosphere of preferably carbon monoxide or alternatively hydrogen, nitrogen, or argon to prevent copper, silver, and tellurium or selenium from oxidizing; and the system should be sealed to prevent loss of tellurium or selenium which vaporize readily. The melted mixture of elements should be kept melted and agitated periodically over a period of 2 to 20 hours to insure complete mixing and reaction. Freezing of the molten melt should be accomplished under a partial vacuum, such as a vacuum in which the pressure is about one-inch mercury, to suppress the unusually high gas solubility in the liquid metal. High gas solubility in the ingot leads to a porous casting which typically results in poorer thermoelectric and mechanical properties. After the alloy composition has been placed in the mold and solidified, further cooling can be carried out under pressure in an atmosphere of a heavy gas such as argon or carbon dioxide to insure a more uniform rate of cooling of the ingot. The alloy should be allowed to cool at a slow rate in a furnace, rather than by a quenching operation, to prevent the formation'of stresses in the ingot. A desired cooling rate is one of approximately a few degrees centigrade per minute. The melting and casting can be carried out in crucibles of such inert materials as carbon, alumina, pre-fired lavite, and quartz.

A less preferred procedure for forming thermoelectric legs is to pulverize the original ingots of copper-silver-tellurium or copper-silver-selenium alloy, press the pulverized alloy into the desired configuration, and then sinter. Under the presently used pressing techniques, however, thermoelectric legs of noncontinuous structure and thus higher electrical resistivity are found to be produced by this method. By contrast, in the casting method described, a dense, uniform, continuous, structure is formed. It is contemplated, however, that improved pressing techniques may become available by which a dense, uniform, continuous structure will be obtained.

After casting, the ingot may be machined to the desired dimensions, if that is necessary, and then should be annealed to relieve stresses and make the composition of the thermoelectric leg more homogeneous. The annealing may be carried out in a sealed quartz tube member as known in the prior art. An especially useful combination has been found to be P-type legs of this invention, particularly those made from selenide or gamma telluride compositions, with legs based on N- type lead telluride, the latter being disclosed in U.S. Pat. Nos. 2,811,440; 2,811,570; and 2,811,571.

The invention will be better illustrated in the following examples.

EXAMPLE 1 A mixture of pulverized copper, silver, and tellurium in equal atomic percentages was charged into a quartz tube under an atmosphere of carbon monoxide. The

under an atmosphere of hydrogen. Temperatures of 650850C. for 12 hours or more are preferred. The resulting elements are quite strong, a gamma telluride alloy composition, for example, having a compressive strength of about 30,000 pounds/square inch at room temperature. The alloys of this invention all have a Knoop hardness number of about 9 5.

1n forming a thermocouple, the P-type components of this invention are best attached to a contacting member at the cold junction by a metallurgical bond. A use- .ful bonding material has been found to be a coppersilver alloy solder having an eutectic temperature of approximately 779C. and comprising 39.9 atomic percent copper and 60.1 atomic percent silver. A bond with this solder withstands the high temperatures preferably used at the cold junction of a component of this invention. Further, at such temperatures there is no migration of elements from the solder into the component to degrade the characteristics of the alloy composition.

Alternatively, tin solder may be used if the'cold junction temperature is maintained below about 180C. The element is soldered to a contact electrode member of a metal such as copper, nickel, or any other good metallic electrical and thermal conductor.

At the hot junction the best contact has been found to be a pressure contact using as the material of the contacting electrode an oxide-free molybdenum, iron, molybdenum-iron alloy, nickel, graphite, or platinum. The most preferable material has been found to be molybdenum.

A complete thermocouple is formed by joining a novel P-type thermoelectric leg to an N-type thermoelectric leg by an interconnecting thermojunction tube was then placed in a vertical tube furnace, and heated to a temperature of 1,000C. where the entire mixture became molten. The melt was maintained at 1,000C. for about 18 hours, agitated periodically to insure complete mixing, and then allowed to cool to room temperature in a furnace at a controlled rate of approximately 2C./minute. The resulting ingot was pulverized and again melted at 1,000C. and cast under a partial vacuum in a cylinder one-quarter-inch in diameter and about one-inch long. After similarly slow cooling to room temperature, the cylinder was annealed at 700C. for 25 hours and then cooled to room temperature at approximately 2C. per minute. A thermoelectric leg 0.15 inch in length was cut from this cylinder with an abrasive saw according to familiar technology.

A thermocouple was then prepared using this thermoelectric leg and a similarly shaped N-type lead telluride leg. Separate copper electrodes were soldered to an end of each of the legs using a copper-silver eutectic solder for the copper silver telluride leg and tin solder for the lead telluride leg. The other end of each of the legs was pressure-contacted with a common molybdenum electrode. With the couple under a protective atmosphere comprising about percent argon and 5 percent hydrogen, the molybdenum-contacted junctions were heated to about 650C. with an electrical resistance heater while the other end was maintained at about 250C. The performance of the copper silver telluride leg was separately monitored and found to produce over 1,700 hours of operation, an average open circuit voltage of 0.095 volts and an average resistance of 0.0064 ohms to give an average calculated matched load power of 0.349 watts.

EXAMPLE 2 A cast thermoelectric leg having a composition of about 31.7 atomic percent copper and about 35 atomic percent silver and about 33.3 atomic percent tellurium and having a diameter of 0.25 inch and length of 0.1 1 inch was included in a thermocouple with a leg of N- type lead telluride by bonding the cold ends of the legs to separate copper electrodes with tin solder and pressure contacting the hot ends with a common electrode of cold-rolled steel. The thermocouple was heated to about 600C. at the hot junction while the cold end was held at about 5C. The copper-silver-telluride element was monitored and found to produce an open circuit voltage of 0.090 volts and a resistance of 0.0032 ohms from which was calculated a matched load power of 0.62 watts.

EXAMPLE 3 A thermoelectric leg having a 0.25 inch diameter and 0.25 inch length was cast from a composition that comprised 44.5 atomic percent copper, 22.2 atomic percent silver, and 33.3 atomic percent tellurium. The leg was joined with a leg of N-type lead telluride by soldering the cold ends of the legs to separate copper electrodes with tin solder and pressure contacting the hot ends with a common electrode of molybdenum. With the hot and cold junctions of the thermocouple maintained at about 625 and 75C. respectively, the copper silver telluride leg was monitored and found to produce 0.069 volts of open circuit voltage and a resistance of 0.0038 ohms from which was calculated 0.31 watts of matched load power.

EXAMPLE 4 A thermoelectric leg having a diameter of 0.36 inch and a length of 0.24 inch, was cast from a composition including about 65.7 atomic percent copper, about 1 atomic percent silver, and about 33.3 atomic percent selenium using the general procedure of Example 1. The leg was included in a thermocouple with a N-type lead telluride leg by bonding the cold ends to separate copper electrodes, using copper-silver solder for the P-type leg and tin solder for the N-type leg. The hot ends of the legs were pressure-contacted with a common electrode of molybdenum. With the hot and cold junctions of the thermocouple maintained at about 620C. and 190C. respectively, the copper silver selenide leg was monitored and found to produce 0.0898 volts of open circuit voltage and a resistance of 0.00437 ohms from which was calculated 0.462 watts of matched load power.

Although the compositions of this invention consist essentially of copper, silver, and either tellurium or selenium, it is recognized that some other elements can be included in the compositions without destroying, and in some combinations to increase, useful thermoelectric conversion properties. Compositions that are basically copper-silver-tellurium or copper-silverselenium compositions, but which include minor, proportions of other elements, are contemplated within this invention. It is further comtemplated within this invention that minor amounts of tellurium and selenium be substituted for one another.

I claim:

1. In a thermoelectric generator, at least one P-type thermoelectric leg or P-type thermoelectric leg segment that consists essentially of copper, silver, and one member of the group tellurium and selenium in proportions defined by one of the two following tables:

1. for tellurium compositions,

a. 32.5 atomic percent tellurium 33.7

atomic percent b. 27 atomic percent copper 66.5 atomic percent 0. 1 atomic percent silver 40 atomic percent;

2. for selenium compositions,

a. 32.5 atomic percent 5 selenium 33.7

atomic percent b. 60 atomic percent copper 66.5 atomic percent c. 1 atomic percent silver 7 atomic percent.

fined in claim 1 that includes between about 60 and 66.5 atomic percent copper.

5. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a selenium composition as defined in claim ll.

6. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a selenium composition as defined in claim 1 in which the amount of the selenium is slightly in excess of the stoichiometric amount of 33.3 atomic percent.

7. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a selenium composition as defined in claim 1 in which the amount of selenium in the composition is slightly in excess of the stoichiometric amount of 33.3 atomic percent, and the composition includes less than 3 atomic percent silver.

8. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a selenium composition as defined inclaim l in which the selenium comprises about 33.5 atomic percent of the composition and the silver comprises about 1 atomic percent of the composition.

copper 35 atomic per- 10. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment is a cast structure.

11. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment is in high-temperature crystal form.

12. In a thermoelectric generator, at least one P-type thermoelectric leg or P-type thermoelectric leg segment that consists essentially of 60 to 66.5 atomic percent copper, l to 7 atomic percent silver, and selenium in an amount slightly in excess of the stoichiometric amount of 33.3 atomic percent up to 33.7 atomic percent, said thermoelectric leg or thermoelectric leg segment having an imperfect lattice structure based on the formula (Cu, Ag) Se, with copper atoms being displaced from the lattice.

13. A thermoelectric generator of claim 12 in which said thermoelectric leg or thermoelectric leg segment is a cast structure.

14. In a thermoelectric generator, at least one P-type thermoelectric leg or P-type thermoelectric leg segment that consists essentially of ingredients selected from copper, silver, and one member of the group tellurium and selenium in proportions defined by one of the two following tables:

1. for tellurium compositions,

a. 32.5 atomic percent atomic percent b. 60 atomic percent copper 67 atomic percent c. atomic percent silver 7 atomic percent;

2. for selenium compositions,

a. 32.5 atomic percent atomic percent b. 60 atomic percent copper cent 0. 0 atomic percent g silver 5 7 atomic percent; said thermoelectric leg or thermoelectric leg segment being a cast structure and having an imperfect lattice structure based on the molecular formula (Cu, Ag) Ch, where Ch is tellurium or selenium, with copper atoms being displaced from the lattice.

15. A thermoelectric generator of claim 14 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a tellurium or a selenium composition in which the amount of the tellurium or selenium is slightly in excess of the stoichiometric amount of 33.3 atomic percent.

16. A thermoelectric generator of claim 14 in which said thermoelectric leg or thermoelectric leg segment is in high-temperature crystal form.

17. For use in a thermoelectric generator, at least one P-type thermoelectric leg or P-type thermoelectric leg segment that consists essentially of copper, silver, and one member of the group tellurium and selenium in proportions defined by one of the two following tables:

tellurium 33 .7

selenium 33.7

67 atomic perl. for tellurium compositions,

a. 32.5 atomic percent tellurium 33.7

atomic percent b. 27 atomic percent copper 66.5 atomic percent c. 1 atomic percent silver 40 atomic percent;

2. for selenium compositions,

a. 32.5 atomic percent selenium 33.7

atomic percent b. 60 atomic percent 5 copper 66.5 atomic percent 0. 1 atomic percent silver 5 7 atomic percent. 18. An alloy composition having useful P-type thermoelectric properties consisting essentially of copper, silver, and one member of the group tellurium and selenium in proportions defined by one of the two following tables:

1. for tellurium compositions,

percent c. 1 atomic percent silver g 7 atomic percent. 19. A selenium composition of claim 18 in which the amount of selenium in the composition is slightly in ex- 'cess of the stoichiometric amount of 33.3 atomic percent, and the composition includes less than 3 atomic percent silver. 

1. IN A THERMOELECTRIC GENERATOR, AT LEAST ONE P-TUPE THERMOELECTRIC LEG OR P-TYPE THERMOELECTRIC LEG SEGMENT THAT CONSISTS ESSENTIALLY OF COPPER, SILVER, AND ONE MEMBER OF THE GROUP TELLURIUM AND SELENIUM IN PROPORTIONS DEFINED BY ONE OF THE TWO FOLLOWING TABLES;
 1. FOR TELLURIUM COMPOSITIONS, A. 32.5 PERCENT TELLURIUM 33.7 ATOMIC PERCENT B. 27 ATOMIC PERCENT COPPER 66.5 ATOMIC PERCENT C. 1 ATOMIC PERCENT SILVER 40 TOMIC PERCENT;
 2. FOR SELENIUM COMPOSITIONS, A. 32.5 ATOMIC PERCENT SLELNIUM 33.7 ATOMIC PERCENT B. 60 ATOMIC PERCENT COPPER 66.5 ATOMIC PERCENT C. 1 ATOMIC PERCENT SILVER 7 ATOMIC PERCENT.
 2. for selenium compositions, a. 32.5 atomic percent < or = selenium < or = 33.7 atomic percent b. 60 atomic percent < or = copper < or = 66.5 atomic percent c. 1 atomic percent < or = silver < or = 7 atomic percent.
 2. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a tellurium composition as defined in claim 1 that includes between about 30 and 40 atomic percent copper.
 2. for selenium compositions, a. 32.5 atomic percent < or = selenium < or = 33.7 atomic percent b. 60 atomic percent < or = copper < or = 66.5 atomic percent c. 1 atomic percent < or = silver < or = 7 atomic percent.
 2. for selenium compositions, a. 32.5 atomic percent < or = selenium < or = 33.7 atomic percent b. 60 atomic percent < or = copper < or = 66.5 atomic percent c. 1 atomic percent < or = silver < or = 7 atomic percent.
 2. for selenium compositions, a. 32.5 atomic percent < or = selenium < or = 33.7 atomic percent b. 60 atomic percent < or = copper < or = 67 atomic percent c. 0 atomic percent < or = silver < or = 7 atomic percent; said thermoelectric leg or thermoelectric leg segment being a cast structure and having an imperfect lattice structure based on the molecular formula (Cu, Ag)2Ch, where Ch is tellurium or selenium, with copper atoms being displaced from the lattice.
 3. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a tellurium composition as defined in claim 1 that includes between about 43 and 50 atomic percent copper.
 4. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a tellurium composition as defined in claim 1 that includes between about 60 and 66.5 atomic percent copper.
 5. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a selenium composition as defined in claim
 1. 6. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a selenium composition as defined in claim 1 in which the amount of the selenium is slightly in excess of the stoichiometric amount of 33.3 atomic percent.
 7. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a selenium composition as defined in claim 1 in which the amount of selenium in the composition is slightly in excess of the stoichiometric amount of 33.3 atomic percent, and the composition includes less than 3 atomic percent silver.
 8. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a selenium composition as defined in claim 1 in which the selenium comprises about 33.5 atomic percent of the composition and the silver comprises about 1 atomic percent of the composition.
 9. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a tellurium composition as defined in claim 1 in which the ingredients are included in the following proportions: 33.2 atomic percent < or = tellurium < or = 33.5 atomic percent 31 atomic percent < or = copper < or = 35 atomic percent 32 atomic percent < or = silver < or = 36 atomic percent.
 10. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment is a cast structure.
 11. A thermoelectric generator of claim 1 in which said thermoelectric leg or thermoelectric leg segment is in high-temperature crystal form.
 12. In a thermoelectric generator, at least one P-type thermoelectric leg or P-type thermoelectric leg segment that consists essentially of 60 to 66.5 atomic percent copper, 1 to 7 atomic percent silver, and selenium in an amount slightly in excess of the stoichiometric amount of 33.3 atomic percent up to 33.7 atomic percent, said thermoelectric leg or thermoelectric leg segment having an imperfect lattice structure based on the formula (Cu, Ag)2Se, with copper atoms being displaced from the lattice.
 13. A thermoelectric generator of claim 12 in which said thermoelectric leg or thermoelectric leg Segment is a cast structure.
 14. In a thermoelectric generator, at least one P-type thermoelectric leg or P-type thermoelectric leg segment that consists essentially of ingredients selected from copper, silver, and one member of the group tellurium and selenium in proportions defined by one of the two following tables:
 15. A thermoelectric generator of claim 14 in which said thermoelectric leg or thermoelectric leg segment consists essentially of a tellurium or a selenium composition in which the amount of the tellurium or selenium is slightly in excess of the stoichiometric amount of 33.3 atomic percent.
 16. A thermoelectric generator of claim 14 in which said thermoelectric leg or thermoelectric leg segment is in high-temperature crystal form.
 17. For use in a thermoelectric generator, at least one P-type thermoelectric leg or P-type thermoelectric leg segment that consists essentially of copper, silver, and one member of the group tellurium and selenium in proportions defined by one of the two following tables:
 18. An alloy composition having useful P-type thermoelectric properties consisting essentially of copper, silver, and one member of the group tellurium and selenium in proportions defined by one of the two following tables:
 19. A selenium composition of claim 18 in which the amount of selenium in the composition is slightly in excess of the stoichiometric amount of 33.3 atomic percent, and the composition includes less than 3 atomic percent silver. 